The fuel costs in limestone burning are considerable and may amount to more than 50% of the production costs. Since coal is in most cases the most cost-effective energy carrier, most lime kilns are nowadays fired with coal. However, coal may be very different in terms of its properties. A high content of ash and sulphur is thus disadvantageous since the unhydrated lime is contaminated in an undesirable manner.
A particular feature of coal is its swelling index. From numerous practical tests, it is known that coal with a swelling index of more than 1.0 could previously not be used in lime kilns, in particular in co-current/counter-current regenerative lime kilns, since serious operational problems, such as, for example, clogging of the combustion lances, occurred. Owing to the limited usability of such coal in co-current/counter-current regenerative lime shaft kilns, either a rotary tubular kiln was previously used or petrol coke or liquid or gaseous fuels had to be used instead. However, rotary tubular kilns have the considerable disadvantage that they use from 50-100% more fuel than co-current/counter-current regenerative lime kilns and consequently considerably more CO2 is thereby discharged into the atmosphere. The use of petrol coke is also disadvantageous since its sulphur content can be up to 10 times higher than that of coal, whereby other limitations are produced. Furthermore, liquid and gaseous fuels may not be available under some circumstances or may be significantly more expensive than coal.
DE 32 27 395 C2 discloses a method for producing briquettes for the fixed bed of a gasification reactor or shaft kiln, from 30 to 85% of non-caking raw coal and from 15 to 70% of the caking raw coal being mixed and subsequently formed into briquettes, the caking coal having a swelling index of ≧5.